1. Field of the Invention
The present invention relates to a camera automatic focusing device which drives the camera objective lens to its in-focus or best-focus position by a single reversible motor and more particularly to apparatus to prevent any overload conditions.
2. Description of the Prior Art
In a camera system such as of a single lens reflex camera wherein the objective lens is changeable, various lenses such as a wide angle lens, a standard lens, telephoto lens, macro lens, etc. are adapted to be selectively and detachably mounted on a single camera body. The lenses each have closest focusable distances determined from their constructions and which are not necessarily the same. Moreover, the lenses have different amounts of the movement of their focusing lens elements for focusing, i.e. the amounts of the movement in the focusable range from the closest focusing position to the infinity focusing position. Picture taking or photographing lenses are usually constructed such that when the focusing lens element adapted to be moved along the optical axis for focusing reaches the closest focusing position, the element is prevented from moving further by a prevention mechanism provided in a lens barrel. The focusing lens element is also prevented from moving beyond the infinity focusing position by a prevention mechanism provided in a lens barrel. In an automatic focusing system wherein the focusing lens element arranged as mentioned above in a picture taking lens is adapted to be driven, for automatic focusing, along the optical axis of the lens by an electric motor mounted within the camera body, certain problems relating to an overload condition can occur. When a target object on which the picture taking lens are to be focused is at a distance closer to the camera than the closest focusable distance, the motor in the camera body still tends to drive the focusing lens element beyond the closest focusing position. As a result, the focusing lens element remains stopped at the closest focusing position, while the motor is forced to stop by the prevention mechanism or continues to be driven with a sliding clutch mechanism between the motor and the focusing lens element. In both cases, the load to the motor is increased by the stoppage of the focusing lens element and electrical current continues to flow into the motor while the focusing lens element is being stopped, resulting in such problems as motor damage and a large consumption of power from a motor driving power source. A similar situation will occur when an object to be focused is far position beyond the furthest focusable distance adapted for close-up photography wherein an intermediate ring is mounted between a camera body and a picture taking lens or a close-up lens is mounted at the front of a picture taking lens.